1. Field of the Invention
The present invention relates to flight vehicle control systems and methods and, more particularly, to a flight vehicle control system and method for controlling flight vehicle performance in which pilot commands are combined with a selected set of sensor measurements to produce appropriate flight control responses.
2. Description of the Related Art
It is well known in the art that the performance of flight vehicles such as aircraft can be significantly improved where automatic control is used to translate pilot commands such as stick force or control column force into flight control actions, such as positioning of aerodynamic control surfaces.
Similarly, closed loop control systems and methods which use flight vehicle performance data (typically derived from sensors) to automatically adjust flight control parameters, such as in positioning of aerodynamic control surfaces, are also known to improve flight performance. These systems and methods, often referred to as automatic pilots, operate independently of pilot commands.
Attempts have been made in the past, with varying degrees of success, to devise control systems and methods which combine pilot commands with automatic closed loop control systems to produce an integrated response capable of controlling flight vehicle performance throughout the various phases of flight. However, these systems and methods have sometimes suffered from limited or poor response characteristics.
The state of the prior art in flight vehicle control technology, as summarized above, must be compared with recent trends in flight vehicle designs which place significantly greater demands and reliance on automatic controller designs. To illustrate, it is well known in the art that a number of favorable aircraft performance characteristics, e.g. improved maneuverability and greater fuel efficiency, are achieved by moving the center of gravity range toward the rear of the aircraft. Improved aircraft performance characteristics are also achievable by decreasing the size of certain control surfaces. For example, weight and trim drag can be reduced by decreasing tail size. Unfortunately, introduction of these design features increases aircraft instabilities. The frequencies of these instabilities are often such that the pilot is incapable of providing control inputs through the stick or control column to compensate. Thus, automatic control systems and methods with enhanced capabilities are needed to offset the increased stability problems of less inherently stable aircraft designs.
In the past, a number of approaches have been used in the design of flight control systems and methods and the control laws which they incorporate. Approaches range from simple damped systems which respond to short period perturbations to more complex systems that include pilot relief modes. Pilot relief modes are control features such as axis rotation rate commands with attitude or angle hold that allow the pilot, for example, to command the vehicle to climb at a constant rate of one thousand feet per minute.
These latter systems and methods provide reasonably good stability for unstable as well as stable flight vehicle designs. However, they have suffered from a number of objectionable characteristics. For example, following a momentary pilot command input, the control system or method may not automatically return the vehicle to its previously commanded trim conditions, such as steady horizontal flight or a constant climb rate.
These systems and methods may also have undesirable response characteristics when the vehicle experiences transient conditions such as windshear, speedbrake deployment, engine failure, or other disturbances to stable flight. For example, an aircraft controlling pitch attitude or flight path angle through the elevator will reduce speed and increase the angle of attack when encountering downdrafts or engine failures. This is a significant safety problem when flying at low speeds.
Thus, control systems and methods known in the art have generally failed to provide a capability for rapid, tailored, and integrated response to both pilot and selected sensor inputs while maintaining invariant handling qualities and stability characteristics for flight vehicles having a wide range of center of gravity locations and operating conditions.
It is an object of the present invention to provide a flight vehicle control system and method having rapid, tailored, and integrated response to pilot and sensor inputs, invariant or task-tailored handling qualities, and favorable stability characteristics for vehicles having a wide range of center of gravity locations.
It is further an object of the present invention to provide a flight vehicle control system and method in which pilot command input gradients (e.g. column force per g and column force per knot) are relatively invariant with respect to location of the center of gravity and are tailored to the flight phase.
It is still further an object of the present invention to provide a control law concept for uses including application to flight vehicles with unstable designs that furnishes maneuvering and trim characteristics as good as or better than those for current stable flight vehicle designs.
It is further an object of the present invention to provide a control law concept in which gain scheduling as a function of center of gravity location or stabilizer position is not required.
Additional objects and advantages of the invention will be set forth in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.